(Supported in part by CNRS grants to M. Bornens) In this ongoing study of the structure of the human centrosome/centriole complex is being studied by various methods including IVEM tomography of isolated centrioles that are embedded in negative stain. Because centrioles are extremely small (0.25 by 0.5 (m) and complex, the only way their morphology can be studied is with EM. Normally, these organelles are examined in sections of conventionally plastic embedded specimens. The resolution obtained using this technique is somewhat limited. Dr. Ruiz has isolated human centrosomes and has been studying them after negative staining. She has obtained high-resolution images showing the periodicity of the centriolar structure. However, to study the 3-D structure she needed the BMIRR's IVEM automated tomography system because of the thickness of the whole-mount preparations and the dose-sensitivity of the negative-stain preparations. While in residence she also explo red the u tility of conducting tomography on frozen-hydrated centrioles. On her visit Dr. Ruiz recorded six double-tilt series and one single-tilt tomographic series. A test reconstruction was made by Dr. Rath. This project will continue with the assistance of Dr. Radermacher and was abstracted this year: Ruiz, T., M. Radermacher, B. Rath, C. Rieder and M. Bornens (1998) The centrosome: three-dimensional structure of a cell organelle at 40A resolution obtained by electron-tomography. Microscopy and Microanalysis 4:436-437.